The three projects to be described here represent a continuation of our work on the normal and altered oral mucosa. Two of the projects use animal models of conditions in naturally occurring lesions. Work on the hyperplastic lesion of zinc deficiency concentrates on changes in lipids and intercellular substance which might be responsible for the defective desquamation seen in this lesion. Experiments on diminished food intake and liquid diet are designed to simulate conditions in the history of the aged patient leading to ulcerative and atrophic change. The third project examines leukoplakias associated with different uses of tobacco and is designed to establish histologic criteria for the prognosis of the malignant potential of leukoplakias. Methods to be used: 1. Biochemical assay of lipids using standard techniques. 2. Electron microscopical examination of tissue using routine and special staining methods. 3. Histological and histochemical methods. 4. Quantitation at both light and electron microscopic levels using methods of morphometry and stereology.